When drilling with casing, the volume of drilling fluid in the drill string is large. If a compressible fluid such as, for example, air is used as a drilling fluid, significant handling problems can occur with this large volume of fluid under pressure.
When drilling, compressed gas has to be introduced through the ID of the top drive that is connected to the top end of the casing drill string. When the top drive is required to pick up a next joint of casing, the top drive is removed from the casing string ID. During that time if the drill string is left open, compressed gas can exit under considerable pressure, which would prevent a connection from being made until the pressure in the casing drill string is equalized with the atmospheric pressure. Time required for the equalization of pressure would be long and at the same time energy introduced, to compress the gas in the first place, would be lost.
When the connection of the next joint of casing was finally made, the gas in the whole length of the drill string would have to be compressed again prior to resuming the drilling operation.
As such, for drilling with casing with compressed gas as the drilling fluid to be economically feasible, the volume of the compressed gas has to be retained in the drill string throughout the drilling process, even when the top drive is disconnected in order to attach new joint of the casing string.